Magnetoelastic torque tool

ABSTRACT

A hand tool for applying torque to a fastener includes a portion for engaging the fastener, a handle to which torque is hand-applied and a torque carrying member operatively coupled with the handle and the fastener-engaging portion for transmitting the hand-applied torque to the fastener. The torque carrying member includes a magnetoelastic torque transducer comprising a ferromagnetic, magnetostrictive region affixed to, associated with or forming a part of the surface of the torqued member for altering in magnetic premeability in response to the application of torque to the member. Additionally, the transducer includes a magnetic source for applying a cyclically time varying magnetic field to the ferromagnetic, magnetostrictive region for sensing the change in permeability caused by the applied torque, apparatus/circuitry for converting the sensed change in permeability to an electrical signal indicative of the magnitude of the torque applied to the member and an indicator responsive to the electrical signal for providing an indication that a predetermined torque has been applied to the fastener. The ferromagnetic, magnetostrictive region is advantageously formed of nickel maraging steel, desirably 18% Ni maraging steel. Preferably, the transducer comprises a pair of axially spaced-apart annular bands defined within the ferromagnetic, magnetostrictive region, the bands being endowed with residual stress created, respectively symmetrical right and left hand helically directed magnetic anisotropy of sufficiently large magnitude that the contribution to total magnetic anistropy of any random anisotropy in the member is negligible.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.938,404, filed December 5, 1986, now U.S. Pat. No. 4,760,745.

TECHNICAL FIELD

The present invention relates to torque sensors and, more particularly,to non-contacting magnetoelastic torque transducers for providing ameasure of the torque applied to a rotary shaft.

BACKGROUND ART

In the control of systems having rotating drive shafts, it is generallyrecognized that torque is a fundamental parameter of interest.Therefore, the sensing and measurement of torque in an accurate,reliable and inexpensive manner has been a primary objective of workersfor several decades. Although great strides have been made, thereremains a compelling need for inexpensive torque sensing devices whichare capable of continuous torque measurements over extended periods oftime despite severe environments.

All magnetoelastic torque transducers have two features in common--(1) atorqued member which is ferromagnetic and magnetostrictive, the formerto ensure the existence of magnetic domains and the latter to allow theorientation of the magnetization within each domain to be altered by thestress associated with applied torque; and (2) a means, most usually butnot necessarily electromagnetic means, for sensing variations from theuntorqued distribution of domain orientations. The differences among thevarious existing or proposed magnetoelastic torque transducers lie inthe detailed variations of these common features.

It is well known that the permeability of magnetic materials changes dueto applied stress. When a torsional stress is applied to a cylindricalshaft of magnetostrictive material, each element in the shaft issubjected to a shearing stress. This shearing stress may be expressed interms of a tensile stress and an equal and perpendicular compressivestress with the magnitude of each stress being directly proportional tothe distance between the shaft axis and the element. The directions ofmaximum tension and compression occur along tangents to 45° left-handedand 45° right-handed helices about the axis of the shaft. The effect ofthe torque is to increase the magnetic permeability in directionsparallel to one of the helices and, correspondingly, to decrease themagnetic permeability in directions parallel to the other of thehelices. In their article "Magnetic Measurements of Torque in a RotatingShaft", The Review of Scientific Instruments, Vol. 25, No. 6, June,1954, Beth and Meeks suggest that in order to use permeability change asa measure of the applied torque, one should monitor permeability alongthe principal stress directions and pass the magnetic flux through theshaft near its surface. This is because the stress is greater, thefurther the element is from the shaft axis and it is along the principalstress directions that the maximum permeability change is expected. Toaccomplish this, Beth and Meeks used a yoke carrying a driving coil forproducing an alternating flux in the shaft and pickup coils on each ofseveral branches to detect the permeability changes caused by theapplied torque in flux paths lying in or near the principal stressdirections in the shaft. When the shaft is subjected to a torque, themechanical stresses attributable to torque resolve into mutuallyperpendicular compressive and tensile stresses which cause thepermeability in the shaft to increase in the direction of one stress anddecrease in the direction of the other. As a result, the voltage inducedin the pickup or measuring coils increases or decreases. The differencein magnitude of the induced voltages is proportional to the torsionalstress applied to the shaft. A similar approach was taken in U.S. Pat.No. 3,011,340--Dahle. The principal shortcoming in these type devices isthe need to accomplish permeability sensing along the principal stressdirections with its attendant disadvantages, such as its sensitivity tovariations in radial distance from the shaft, magnetic inhomogeneityaround the shaft circumference and non-compensatable dependence on shaftspeed. As a result, devices such as these have only found applicationson large diameter shafts, i.e., 6-inches and larger, but have not beenfound to be adaptable to smaller shafts where the vast majority ofapplications exist.

It was felt by some that devices such as were taught in Beth and Meeksand U.S. Pat. No. 3,011,340--Dahle, wherein the rotating shaft itselfacted as the magnetic element in the transducer, had significantdrawbacks in practical application. This is because the materials andmetallurgical processing which may have been used to impart the desiredmechanical properties to the shaft for its desired field of use will, inmost cases, not be optimum or even desirable for the magnetic qualitiesrequired in a magnetoelastic torque sensor. The random anisotropy in ashaft created during its manufacture, due to internal stresses and/orresulting from regions of differing crystal orientation will causelocalized variations in the magnetic permeability of the shaft whichwill distort the desired correlation between voltage sensed and appliedtorque. The solution, according to U.S. Pat. No. 3,340,729--Scoppe is torigidly affix, as by welding, a magnetic sleeve to the load-carryingshaft so that a torsional strain proportional to the torsional load isimparted to the sleeve. The measuring device employed now sensespermeability changes in the rotating sleeve rather than in the rotatingshaft. This permits, according to Scoppe, a material to be selected forthe shaft which optimizes the mechanical and strength propertiesrequired for the shaft while a different material may be selected forthe sleeve which optimizes its magnetic properties. As with prior artdevices, the Scoppe torquemeter utilized a primary winding forgenerating a magnetic flux and two secondary windings, one oriented inthe tension direction and the other in the compression direction.Although obviating at least some of the materials problems presented byDahle, the use of a rigidly affixed sleeve creates other, equallyperplexing problems. For example, the task of fabricating and attachingthe sleeve is a formidable one and even when the attachment means iswelding, which eliminates the bond strength problem, there remains thevery significant problem that the coefficient of thermal expansion ofthe steel shaft is different (in some cases up to as much as 50%greater) than the corresponding coefficient of any magnetic materialselected for the sleeve. A high temperature affixing process, such aswelding, followed by cooling establishes stresses in the magneticmaterial which alters the resultant magnetic anisotropy in anuncontrolled manner. Moreover, annealing the shaft and sleeve to removethese stresses also anneals away desirable mechanical properties in theshaft and changes the magnetic properties of the sleeve. Furthermore,like the Dahle device, the shortcomings of Scoppe's transducer, due toits need to monitor permeability changes lying along the principalstress directions, are its sensitivity to variations in its radialdistance from the shaft, magnetic inhomogeneity around the shaftcircumference and dependence on shaft speed.

A different approach to magnetoelastic torque sensing utilizes thedifferential magnetic response of two sets of amorphous magnetoelasticelements adhesively attached to the torqued shaft. This approach has theadvantage over prior approaches that it is insensitive to rotationalposition and shaft speed. However, it requires inordinate care in thepreparation and attachment of the elements. Moreover, transducerperformance is adversely affected by the methods used to conform theribbon elements to the shape of the torqued member; the properties ofthe adhesive, e.g., shrinkage during cure, expansion coefficient, creepwith time and temperature under sustained load; and, the functionalproperties of the amorphous material itself, e.g., consistency,stability. Still another concern is in the compatibility of the adhesivewith the environment in which the transducer is to function, e.g., theeffect of oil, water, or other solvents or lubricants on the propertiesof the adhesive.

In the article "A New Torque Transducer Using Stress Sensitive AmorphousRibbons", IEEE Trans. on Mag., MAG-18, No. 6, 1767-9, 1982, Harada etal. disclose a torque transducer formed by gluing two circumferentialstress-sensitive amorphous ribbons to a shaft at axially spaced apartlocations. Unidirectional magnetoelastic magnetic anisotropy is createdin each ribbon by torquing the shaft in a first direction before gluinga first ribbon to it, releasing the torque to set-up stresses within thefirst ribbon, torquing the shaft in the opposite direction, gluing thesecond ribbon to it, and then releasing the torque to set-up stresseswithin the second ribbon. The result is that the anisotropy in oneribbon lies along a right-hand helix at +45° to the shaft axis while theanisotropy in the other ribbon lies along an axially symmetric left-handhelix at -45° to the shaft axis. AC powered excitation coils and sensingcoils surround the shaft making the transducer circularly symmetric andinherently free from fluctuation in output signal due to rotation of theshaft. In the absence of torque, the magnetization within the tworibbons will respond symmetrically to equal axial magnetizing forces andthe sensing coils will detect no difference in the response of theribbons. However, when torque is applied, the resulting stressanisotropy along the principal axes arising from the torque combinesasymmetrically with the quiescent anisotropies previously created in theribbons and there is then a differing response of the two ribbons toequal axial magnetizing force. This differential response is a functionof the torque and the sensing coils and associated circuitry provide anoutput signal which is proportional to the applied torque. Utilizingsubstantially the same approach, in Japanese patent publication No.58-9034, two amorphous ribbons are glued to a shaft and symmetricalmagnetic anisotropy is given to the ribbons by heat treatment in amagnetic field at predetermined equal and opposite angles. Amorphousribbons have also been glued to a shaft in a ±45° chevron pattern, seeSasada et al., IEEE Trans. on Mag., MAG-20, No. 5, 951-53, 1984, andamorphous ribbons containing parallel slits aligned with the ±45°directions have been glued to a shaft, see, Mohri, IEEE Trans. on Mag.,MAG-20, No. 5, 942-47, 1984, to create shape magnetic anisotropy in theribbons rather than magnetic anisotropy due to residual stresses. Otherrecent developments relevant to the use of adhesively attached amorphousribbons in a magnetoelastic torque transducer are disclosed in U.S. Pat.No. 4,414,855--Iwasaki and U.S. Pat. No. 4,598,595--Vranish et al.

More recently, in apparent recognition of the severe shortcomingsinherent in using adhesively affixed ribbons, plasma spraying andelectrodeposition of metals over appropriate masking have been utilized.See: Yamasaki et al, "Torque Sensors Using Wire ExplosionMagnetostrictive Alloy Layers", IEEE Trans. on Mag., MAG-22, No. 5,403-405 (1986); Sasada et al, "Noncontact Torque Sensors Using MagneticHeads and Magnetostrictive Layer on the Shaft Surface--Application ofPlasma Jet Spraying Process", IEEE Trans. on Mag., MAG-22, No. 5,406-408 (1986).

The hereinbefore described work with amorphous ribbons was not the firstappreciation that axially spaced-apart circumferential bands endowedwith symmetrical, helically directed magnetic anisotropy contributed toan improved torque transducer. USSR Certificate No. 667,836 discloses amagnetoelastic torque transducer having two axially spaced-apartcircumferential bands on a shaft, the bands being defined by a pluralityof slots formed in the shaft in a ±45° chevron pattern, and a pair ofexcitation and measuring coil-mounting circumferential bobbins axiallylocated along the shaft so that a band underlies each bobbin. The shapeanisotropy created by the slots is the same type of magneticpreconditioning of the shaft as was created, for example, by thechevron-patterned amorphous ribbons of Sasada et al and the slittedamorphous ribbons of Mohri, and suffers from many of the sameshortcomings. USSR Certificate No. 838,448 also discloses amagnetoelastic torque transducer having two spaced-apart circumferentialbands on a shaft, circumferential excitation coils and circumferentialmeasuring coils surrounding and overlying the bands. In this transducerthe bands are formed by creating a knurl in the shaft surface with thetroughs of the knurl at ±45° angles to the shaft axis so that thetroughs in one band are orthogonal to the troughs in the other band. Theknurls are carefully formed by a method which ensures the presence ofsubstantial unstressed surface sections between adjacent troughs so thatthe magnetic permeability of the troughs is different from the magneticpermeability of the unstressed areas therebetween. Inasmuch as thetrough width-to-pitch ratio corresponds to the stressed to unstressedarea ratio and the desired ratio appears to be 0.3, there is nocircumferential region in either band which is intentionally stressedover more than 30% of its circumferential length. This very minimalstress anisotropic preconditioning is believed to be too small toprovide a consistent transducer sensitivity, as measured by theelectronic signal output of the measuring coils and their associatedcircuitry, for economical commercial utilization.

Notwithstanding their many shortcomings in forming sensitive andpractical bands of magnetic anisotropy on a torqued shaft, the effortsevidenced in the Harada et al, Sasada et al, Mohri and Yamasaki et alarticles and the USSR certificates represent significant advances overthe earlier work of Beth and Meeks, Dahle and Scoppe in recognizing thata pair of axially spaced-apart, circumferential bands of symmetrical,helically directed anisotropy permits averaging axial permeabilitydifferences over the entire circumferential surface. This is notablysimpler than attempting to average helical permeability differencessensed along the principal stress axes, as had earlier been suggested.Moreover, neither rotational velocity nor radial eccentricitysignificantly influence the permeability sensed in this manner.Nevertheless, these efforts to perfect means of attachment ofmagnetoelastically optimized material to the surface of the torquedmember introduces unacceptable limitations in the resulting torquesensor. The application to the shaft of adhesively affixed amorphousribbons suffers from significant drawbacks, such as the methods used toconform the ribbons to the shaft, the properties of the adhesive and thefunctional properties of the amorphous material, which make such ribbonsimpractical for commerical implementation. The use of rigidly affixedsleeves as taught by Scoppe and, more recently, in U.S. Pat. No.4,506,554--Blomkvist et al, is unsuitable for practical applications dueto the higher costs involved as well as the stresses created by hightemperature welding and/or the uncertainties in magnetic and mechanicalproperties created by subsequent annealing. Likewise, reliance uponshape anisotropy or predominantly unstressed regions to create stressanisotropy present significant problems which make such techniquesimpractical for commercial implementation.

It is, therefore, apparent that despite the many advances in torquetransducer technology, there still exists a need for a magnetoelastictorque transducer which is significantly more economical than previoustorque transducers, allowing use in many applications for which suchtransducers were not heretofore either economically or environmentallyviable, and which is applicable to small as well as large diametershafts, whether stationary or rotating at any practical speed.

DISCLOSURE OF THE INVENTION

In accordance with one broad aspect of the present invention there isprovided a magnetoelastic torque transducer for providing an electricalsignal indicative of the torque applied to a member in which aferromagnetic and magnetostrictive region of the torqued member servesas a part of the magnetic sensing circuit of the transducer by providingat the surface of said region a pair of axially spaced-apart annularbands endowed with residual stress created, respectively symmetrical,left and right hand helically directed magnetic anisotropy of relativelylarge magnitude, which anisotropy overwhelms and/or renders negligibleor insignificant any random anisotropy in the member as a result ofinternal stresses due to mechanical working, inhomogeneities, crystalorientation, and the like.

In accordance with another aspect of the present invention, there isprovided a magnetoelastic torque transducer for providing an electricalsignal indicative of the torque applied to a member, said member havinga ferromagnetic and magnetostrictive region, said transducer comprisinga pair of axially spaced-apart annular bands defined within said region,said bands having, at least at the surface of said member, respectivelysymmetrical right and left hand helically directed residual stresscreated magnetic anisotropy, each said band having at least onecircumferential region which is free of residually unstressed areas,i.e., said at least one circumferential region is residually stressed,over at least 50% of its circumferential length; means for applying analternating magnetic field to said bands; means for sensing the changein permeability of said bands caused by said applied torque; and meansfor converting said sensed change in permeability to an electricalsignal indicative of the magnitude of the torque applied to said member.In a preferred aspect, the ferromagnetic and magnetostrictive region isformed of nickel maraging steel.

In accordance with another aspect, the present invention contemplates amagnetoelastic torque transducer for providing an electrical signalindicative of the torque applied to a member, including ferromagnetic,magnetostrictive means rigidly affixed to, associated with or forming apart of the surface of said torqued member for altering in magneticpermeability in response to the application of torque to said member,means for applying a magnetic field to said ferromagnetic,magnetostrictive means, means for sensing the change in permeabilitycaused by said applied torque and means for converting said sensedchange in permeability to an electrical signal indicative of themagnitude of the torque applied to said member, the ferromagnetic,magnetostrictive means being formed from nickel maraging steel.

In still another aspect of the present invention, there is provided amethod of sensing the torque applied to a member having a ferromagneticand magnetostrictive region, which includes the steps of endowing a pairof axially spaced-apart annular bands within said region withrespectively symmetrical, right and left hand helically directedmagnetic anisotropy, applying an alternating magnetic field to saidbands and sensing the permeability difference between said bandsresulting from the application of torque to said member, the differencebeing indicative of the magnitude of the applied torque, the improvementwhich comprises forming said bands at the surface of said member andendowing said bands with magnetic anisotropy by instilling a residualstress distribution in each band which is sufficiently extensive that atleast one circumferential region within each band is free of residuallyunstressed areas, i.e., said at least one circumferential region isresidually stressed, over at least 50% of its circumferential length. Ina preferred aspect of this method, the ferromagnetic andmagnetostrictive region is formed from nickel maraging steel.

In yet another aspect of the invention, there is provided a method ofsensing the torque applied to a member having a ferromagnetic andmagnetostrictive region which includes the steps of endowing said regionwith helically directed magnetic anisotropy by instilling a residualstress distribution in said region which is sufficiently extensive thatat least one circumferential region within said ferromagnetic andmagnetostrictive region is free of residually unstressed areas, i.e.,said at least one circumferential region is residually stressed, over atleast 50% of its circumferential length, applying an alternatingmagnetic field to said ferromagnetic and magnetostrictive region and toan area of said member not so endowed, and sensing the permeabilitydifference between said ferromagnetic and magnetostrictive region andsaid area resulting from the application of torque to said member, thedifference being indicative of the magnitude of the applied torque. In apreferred aspect of this method, the ferromagnetic and magnetostrictiveregion is formed from nickel maraging steel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a magnetoelastic torque transducer inaccordance with the present invention;

FIG. 2 is a sectional view of a magnetoelastic torque transducer inaccordance with the present invention illustrating one form of magneticdiscriminator useful therewith;

FIG. 3 is a circuit diagram showing the circuitry associated with themagnetic discriminator of FIG. 2;

FIG. 4 is a schematic view of a magnetoelastic torque transducer inaccordance with the present invention illustrating another form ofmagnetic discriminator, and its associated circuitry, useful therewith;

FIG. 5 is a graphical representation of the relationship between appliedtorque and output signal for several magnetoelastic torque transducersof the present invention;

FIG. 6 is a graphical representation of the relationship between appliedtorque and output signal for the magnetoelastic torque transducers ofFIG. 5 after the shafts thereof have been heat treated under identicalconditions;

FIG. 7 is a graphical representation of the general relationship betweentorque transducer sensitivity and residual stress loading along thecircumferential length of a circumferential region of the bands of atransducer of the present invention;

FIG. 8 is an elevational view of a test piece used in torque transducersensitivity testing;

FIG. 9 is a graphical illustration, as in FIG. 7, of the sensitivity vs.residual stress loading relationship for a transducer of the presentinvention wherein the bands thereof were endowed with residual stressinduced magnetic anisotropy by a controlled knurling technique;

FIG. 10 is an exploded view, partially broken away, of one form oftorque tool in accordance with the present invention;

FIG. 11 is an elevational view of the torque tool of FIG. 10; and

FIG. 12 is a circuit diagram of a multivibrator and indicator circuituseful in the torque tool of FIG. 10.

BEST MODE FOR CARRYING OUT THE INVENTION

In accordance with the present invention there is provided amagnetoelastic torque transducer comprising (1) a torque carrying memberat least the surface of which, in at least one complete circumferentialregion of suitable axial extent, is appropriately ferromagnetic andmagnetostrictive; (2) two axially distinct circumferential bands withinthis region or one such band in each of two such regions that areendowed with respectively symmetrical, helically directed residualstress induced magnetic anisotropy such that, in the absence of torque,the magnetization tends to be oriented along a left-hand (LH) helix inone band and along an axially symmetrical right-hand (RH) helix in theother band; and (3) a magnetic discriminator device for detecting,without contacting the torqued member, differences in the response ofthe two bands to equal, axial magnetizing forces.

These features of the magnetoelastic torque transducer of the presentinvention will be better understood by reference to FIG. 1 in which acylindrical shaft 2 formed of ferromagnetic and magnetostrictivematerial or, at least having a ferromagnetic and magnetostrictive region4, is illustrated having a pair of axially spaced-apart circumferentialor annular bands 6,8 endowed with respectively symmetrical, helicallydirected magnetic stress anisotropy in the angular directions ±θ of therespective magnetic easy axes 10,12. A magnetic discriminator 14 isspaced from shaft 2 by a small radial space. In the absence of appliedtorque the magnetization within the bands 6,8 will respond symmetricallyto the application of equal axial magnetizing forces. Longitudinal oraxial components of the magnetization within these two bands remainidentical, since cos θ=cos-(θ) for all values of θ, and the magneticdiscriminator will therefore, detect no difference or zero. With theapplication of torque to shaft 2, the stress anisotropy arisingtherefrom combines asymmetrically with the quiescent anisotropiesintentionally instilled in the bands and there is then a differingresponse of the two bands to equal axial magnetizing force. Since thestress anisotropy is a function of the direction and magnitude of thetorque, the differential response of the two bands will be a monotonicfunction of the torque. The resulting differences in magnetic anisotropyin each of the bands is evidenced by the axial permeability of one bandincreasing and that of the other band decreasing. The difference inaxial permeabilities of the two bands is used to sense the torque. Aproperly designed magnetic discriminator will detect detailed featuresof the differential response and provide an output signal that is ananalog of the torque.

In accordance with the present invention, the torque carrying member isprovided with two axially spaced-apart, distinct circumferential orannular bands in the ferromagnetic region of the member. There are noparticular geometric, space, location or circumferential limitations onthese bands, save only that they should be located on the same diametermember and close enough to one another to experience the same torque.The bands are intentionally endowed with respective symmetrical,helically directed, magnetic anisotropy caused by residual stress.Residual stress may be induced in a member in many different ways, asdiscussed more fully hereinbelow. However, all techniques have in commonthat they apply stress to the member beyond the elastic limit of atleast its surface region such that, when the applied stress is released,the member is unable to elastically return to an unstressed condition.Rather, residual stresses remain which, as is well known, give rise tomagnetic anisotropy. Depending upon the technique utilized for applyingstress, the angular direction of the tangential principal residualstress with the member's axis will vary between values greater than zeroand less than 90°. Preferably, the angular direction of the residualstress and that of the resulting magnetic easy axes, is from 10°-80°and, most desirably, from 20°-60°.

It will be appreciated that inasmuch as the sensing of torque isprimarily accomplished by sensing the change in permeability at thesurface of the torqued member, it is at least at the surface of eachband that there must be magnetic anisotropy created by residual stress.Hence, the limitation that the applied stress must be at leastsufficient to exceed the elastic limit of the member at its surface. Itwill, of course, be appreciated that the application of an appliedstress exceeding the minimum will, depending upon the magnitude of theapplied stress, result in residual stress within the body of the memberas well. For use herein, the term "surface" of the member means at thesurface and within 0.010 inch thereof.

Any method of applying stress to a member to exceed the elastic limitthereof at the surface of the bands may be employed which producesuneven plastic deformation over the relevant cross-section of themember. Thus, the residual stress inducing method may be mechanical,thermal, or any other which is suitable. It is particularly desirablethat the residual stress-inducing applied stress exceed the maximumexpected applied stress when the member is torqued in use. This is toinsure that torquing during use does not alter the residual stresspattern and, thus, the magnetic anisotropy within the bands. Theresidual stress induced in the respective bands should be substantiallyequal and symmetrical in order that axial permeability sensing, whenequal axial magnetizing forces are applied to the member, will produce a"no difference" output in the untorqued condition and equal but oppositeoutput as a result of the application of equal clockwise (CW) andcounter-clockwise (CCW) torques.

The method chosen to apply stress to a member beyond the elastic limitthereof in order to create residual stress is largely a function of themember's size, shape, material and intended application. The method mayinduce continuous and substantially equal residual stresses over theentire surface of the band, i.e., around the entire band circumferenceand along its entire axial length. Alternatively, the method may inducea residual stress pattern within each band which includes both stressedand unstressed areas. Such a pattern, however, is subject to theimportant limitation that each band must have at least one continuouscircumferential region which is free of unstressed areas over at least50% of its circumferential length, desirably over at least 80% of itscircumferential length. In a particularly preferred configuration, eachband would have at least one continuous circumferential region which isfree of unstressed areas over its entire circumferential length. As ageneral matter, it is particularly desirable to maximize the amount ofshaft surface which is intentionally stressed in order to endow as muchof the surface as is possible with relatively large magnitude controlledmagnetic anisotropy. This leaves as little of the shaft surface aspossible subject only to the random anisotropies created during shaftmanufacture, due to internal stresses and resulting from crystalorientation. It should be appreciated that the problems associated withrandom anisotropy inherent in using the shaft itself as an operativeelement, i.e., the sensing region, of the magnetic circuit of the torquesensor are overcome, in accordance with the present invention, byreplacing and/or overwhelming the random anisotropy with relativelylarge magnitude intentionally created residual stress inducedanisotropy. For obvious reasons, the greater the intentionally inducedanisotropy, the less significant is any residual random anisotropy.

As used hereinbefore and hereinafter, the term "circumferential region"means the locus of points defining the intersection of (1) a planepassing perpendicular to the member's axis and (2) the surface of themember, as hereinbefore defined. Where the member is a cylindricalshaft, the circumferential region is a circle defining the intersectionof the cylindrical surface with a plane perpendicular to the shaft axis,and such a circle has a circumference or circumferential length. Statedotherwise, if each element of the member's surface comprising thecircumferential region were examined, it would be seen that each suchelement was either stressed or unstressed. In order to form acommercially functional torque sensor having broad applicability,particularly in small diameter shaft applications, which exhibitsacceptable and commercially reproducible sensitivity, linearity andoutput signal strength, it has been found that at least 50% of theseelements must have been stressed beyond their elastic limit and,therefore, must remain residually stressed after the applied stress isremoved.

The range of methods by which torque carrying members can be endowedwith the desired bands containing residual stress instilled helicallydirected magnetic easy axes, i.e., directions in which magnetization iseasiest, is virtually endless. From the point of view of transducerperformance the most important consideration is the adequacy of theresulting anisotropy, i.e., the band anisotropy created must be at leastof comparable magnitude to the stress anisotropy contributed by theapplied torque. From the point of view of compatibility with the devicein which the transducer is installed, the compelling consideration isconsequential effects on the member's prime function. Other importantconsiderations in selecting a method are practically and economics.Examples of suitable methods for imprinting residual stress inducedmagnetically directional characteristics on, i.e., at the surface of, atorque carrying member include, but are not limited to, torsionaloverstrain; knurling; grinding; mechanical scribing; directed or maskedshot peening or sand blasting; roll crushing; appropriate chemicalmeans; selective heat treatments, e.g., induction, torch, thermal printhead, laser scribing.

Of the foregoing, the creation of areas of residual stress by torsionaloverstrain has been found to be a simple, economical and effectivemethod for small diameter shafts. It is particularly desirable becauseit neither distorts nor interrupts the surface of the shaft and is,therefore, compatible with virtually any application. However, themanner of applying torsional overstrain, e.g., by twisting both sides ofa centrally restrained region, makes it impractical for and inapplicableto large diameter shafts formed of high elastic limit materials.Knurling is a desirable manner of inducing residual stress in a shaft ofvirtually any diameter. With knurling, the exact location of the bands,their axial extent, separation and location can be closely controlled.In addition, knurling allows relatively simple control of the helixangles of the easy axes. Very importantly, knurling permitspredetermination of the salient features of the knurl itself, such aspitch, depth and cross-sectional shape and, thereby, allows control ofthe residual stress induced. It should be appreciated that, inaccordance with the present invention, enough of the surface of eachband must be stressed that there exists within each band at least onecontinuous circumferential region which is free of unstressed areas overat least 50% of its circumferential length. Not all knurling is thisextensive and care must be taken to select a knurl which achieves thisobjective. Inasmuch as knurling disrupts the surface of the shaft inorder to form the knurl thereon, a knurled band is endowed with shapeanisotropy as well as residual stress anisotropy. If it is desired, forexample, for compatibility of the knurled shaft with an intendedapplication, the gross shape features of the knurl may be ground off theshaft to leave only magnetic anisotropy caused by residual stress. Otherforms of cold working, with or without surface deformation, likewisecreate residual stress and associated magnetic anisotropy. In addition,more sophisticated methods, such as electron beam and laser scribing aswell as selective heat treatment can provide the desired anisotropy withless mutilation of the shaft surface than most mechanical cold workingmethods. Moreover, these methods offer the opportunity of very closecontrol of the induced residual stresses by adjustment of the powerdensity and intensity of the beam and/or the thermal gradients.

Whatever method may be selected for creating residual stress within thebands, it should be appreciated that the relationship between thepercent of stressed areas along the circumferential length of acircumferential region within each band ("% stressed areas") andsensitivity (in millivolts/N-M) is one wherein the sensitivity increaseswith increasing "% stressed areas". A plot of these parameters yields acurve which has its greatest slope at the lower values of "% stressedareas" and which has a decreasing slope at the higher values of "%stressed areas", up to 100%, at which point the sensitivity is greatestand the slope is close to zero. The precise shape of the curve, itsslope for any particular value of "% stressed areas", its initial rateof ascent and the point at which the rate of ascent decreases and thecurve levels off are all functions of the material of the bands and themanner in which the stress is applied. A typical curve is shown in FIG.7. At "A", there is no residual stress along the circumferential lengthof the circumferential region. At " C", 100% of the circumferentiallength of the circumferential region is subjected to residual stress."B" represents the approximate point on the curve at which sensitivitybegins to level off, i.e., becomes less responsive to "% stressedareas," a point which is both material and method dependent.

Ideally, torque sensor operation at 100% residual stress, i.e., at "C"on the curve, is best because the rate of change of sensitivity isminimized and the 100% stressed condition is generally easiest to attainwith most methods. As a practical matter, it is difficult to control theresidual stress inducing method to achieve a value for desired "%stressed area" which is less than 100%. However, practical productionproblems aside, acceptable torque sensors can be made which operate atsensitivity levels corresponding to less than 100% residual stress alongthe length of a circumferential region of the bands.

Torque sensor cannot economically and reproducibly be made to operate inthe ascending portion AB along the curve in FIG. 7 since, in thatportion, the sensitivity is extremely responsive to "% stressed areas".This means that even small changes in "% stressed areas" causesrelatively large changes in sensitivity. From a practical, commercialstandpoint, mass produced torque sensors must have a known andreproducible sensitivity. It would be unrealistic to have toindividually calibrate each one. However, even normal productioninconsistencies will cause small "% stressed areas" changes which willresult, in the AB region of the curve, in large sensitivity differencesamong sensors. Therefore, commercially useful torque sensors have tooperate along a flatter portion of the curve, where the slope is closerto zero. Operating in the BC portion of the curve appears to be anacceptable compromise. It is preferred, for most materials and residualstress inducing methods, that the point represented by "B" exceed atleast 50%, preferably at least 80%, stressed areas along thecircumferential length of a circumferential region. This is inrecognition of the fact that the minimum acceptable residual stressloading of a circumferential region is both material and processdependent and that it is generally most desirable to be as close to 100%stress loading as is practical.

To demonstrate the applicability of the foregoing in fabricating anoperable torque sensor, with reference to FIG. 8, a 0.25 inch ODcylindrical shaft 100 was formed with two shoulders 102 of equal axiallength spaced apart by a reduced diameter shaft portion 104 of 0.215inch OD. The shaft was formed of a nickel maraging steel commerciallyavailable as Unimar 300K from Universal-Cyclops Specialty SteelDivision, Cyclops Corporation of Pittsburgh, Pennsylvania and waspre-annealed at 813° C. in hydrogen to relieve internal stresses. Eachshoulder 102 was carefully knurled using a pair of identical 3/4 inchOD, 3/8 inch long knurling rollers having 48 teeth around theircircumference. The shoulders were brought into contact with the knurlingrollers in a controlled manner to form symmetrical knurls on eachshoulder at angles of ±30° to the shaft axis. Careful control of theinfeed of the tool relative to the shoulders allowed the axial width anddepth of each knurl trough to be controlled. The "% stressed areas"along the circumferential length of a circumferential region of eachknurled shoulder was determined by assuming that the knurl trough wasthe only stressed area on the shoulder and that the shoulder surfacebetween troughs was unstressed by the knurling operation; by measuringthe trough width and chordal knurl pitch and converting the chordalpitch to circumferential pitch; and by calculating the trough width tocircumferential pitch ratio, which ratio when multiplied by 100represented the desired "% stressed areas" value. The shaft prepared inthis manner was affixed to a lever arm which permitted 10-one poundweights to be suspended from cables at each end of the arm. The leverarm was so dimensioned that addition or removal of a single one poundweight from either side represented a torque change on the shaft of 0.5N-M. By appropriate shifting of the weights, the torque on the shaftcould be altered in both magnitude and direction.

FIG. 9 graphically illustrates the relationship between "% stressedareas" and sensitivity for a shaft prepared as described hereinabove. Itcan be seen that the curve ascends rapidly up to about 60% stressloading and then appears to level off rather rapidly thereafter. This isbecause there is believed to be a greater correlation at lower "%stressed area" values between the trough width to circumferential pitchratio and the actual percentage of stressed areas along thecircumferential length of a circumferential region of the shaft. As thewidth and depth of the knurling trough increases it becomes apparentthat the shoulder surface between troughs, at least in the vicinity ofthe trough edges, becomes slightly deformed and, more than likely,residually stressed. Therefore, the point on the curve at which 100%stress loading in a circumferential region is actually achieved issomewhat less than the calculated 100% value, accounting for the rapidflattening of the curve at the higher "% stressed areas" portionsthereof. This suggests that, with many processes, such as knurling, the100% stress loading point can be achieved with less than 100%topographic disruption. It will be appreciated in this connection, thateach method of inducing residual stress in a shaft will produce its owndistinctive curve of "% stressed areas" vs. sensitivity, although it isbelieved that each curve will have the same general characteristics asappear in FIGS. 7 and 9.

In accordance with the foregoing, it can be seen that in the absence ofapplied torque, the application to the bands of equal axial magnetizingforces causes the bands to respond symmetrically and the sensing meansassociated with the bands detect no difference in response. When torqueis applied, the principal stresses associated with the applied torquecombine with the residual stresses in the bands in such a manner thatthe resultant stresses in the two bands are different from each other.As a result, the magnetic permeabilities are different and the emfinduced in the sensing means associated with each band reflect thatdifference. The magnitude of the difference is proportional to themagnitude of the applied torque. Thus, the instant system senses adifferential magnetoelastic response to the principal stressesassociated with the applied torque between two circumferential bands.The significance of this is that sensing in this manner amounts tosensing the response averaged over the entire circumference of the band.In this manner, sensitivity to surface inhomogenity, position androtational velocity are avoided.

This sensing of magnetic permeability changes due to applied torque canbe accomplished in many ways, as is disclosed in the prior art. See, forexample, the aforementioned article of Harada et al and U.S. Pat. No.4,506,554. Functionally, the magnetic discriminator is merely a probefor assessing any differential magnetoelastic response to applied torquebetween the two bands. In general, it functions by imposing equalcyclically time varying magnetizing forces on both bands and sensing anydifferences in their resulting magnetization. The magnetizing forces maycome from electrical currents, permanent magnets, or both. Resultingmagnetization may be sensed through its divergence, either by theresulting flux or its time rate of change. The transducer function iscompleted by the electrical circuitry which delivers an electricalsignal that is an analog of the torque.

One method of supplying the magnetization forces and for measuring theresulting difference signal from the sensing coil is shown in FIGS. 2and 3. Referring to FIG. 2, it can be seen that the bands 6,8 aresurrounded by bobbins 16,18 which are concentric with shaft 2. Mountedon bobbins 16,18 are a pair of coils 20,22 and 24,26 of which 22 and 26are excitation or magnetizing coils connected in series and driven byalternating current and 20 and 24 are oppositely connected sensing coilsfor sensing the difference between the fluxes of the two bands. Aferrite material core 28 is optionally provided as a generally E-shapedsolid of revolution. Circumferential gaps 30 between the shaft and theE-shape core are desirably maintained as small and uniform as ispractical to maintain the shaft centered within the core. FIG. 3 showsthat excitation or drive coils 22,26 are supplied in series from ACsource 32 and the emf induced in the oppositely connected sensing coils20,24 is phase sensitively rectified in the rectifier 34 and isdisplayed on voltage display instrument 36. Black dots 38 indicate thepolarity of the coils.

Inasmuch as the stresses in the bands are symmetrical and equal when notorque is applied to shaft 2, under these conditions the output signalfrom the circuitry shown in FIG. 3 will be zero, regardless of theapplied a.c. driving input. This is because the bands have equalmagnetic permeability. Thus the voltages induced in the sensing coilsare equal in magnitude and opposite in polarity and cancel each other.However, when a torque is applied to shaft 2, the respective bands willbe subjected to tensile and compressive stresses, with a resultingincrease of permeability and of the flux passing through one of thebands, and a resulting decrease of permeability and of the flux passingthrough the other of the bands. Thus, the voltage induced in one of thesensing coils will exceed the voltage induced in the other sensing coiland an output signal representing the difference between the inducedvoltages and proportional to the applied torque will be obtained. Thesignal is converted to a direct current voltage in the rectifier 34 andthe polarity of the rectifier output will depend upon the direction,i.e., CW or CCW, of the applied torque. Generally, it has been foundthat in order to obtain linear, strong output signals, the a.c. drivingcurrent should advantageously be maintained in the range 10 to 400milliamperes at excitation frequencies of 1 to 100 kHz.

FIG. 4 illustrates another type of magnetic discriminator for sensingthe permeability change of the bands upon application of a torque to theshaft. Magnetic heads 42,44 comprising a ferromagnetic core and a coilwound thereupon are provided in axial locations along shaft 40 whichcoincide with bands 46,48 and are magnetically coupled to the bands. Themagnetic heads 42,44 are excited by high frequency power source 50through diodes 52,54. With no torque applied to shaft 40, the magneticpermeability of the bands are equal. Therefore, the inductance levels ofboth magnetic heads are equal and opposite in polarity, and the netdirect current output, V_(out), is zero. When torque is applied to shaft40, as shown by arrows 60, the magnetic permeability of one bandincreases while the permeability of the other decreases.Correspondingly, the inductance of one magnetic head increases while theinductance of the other decreases, with a resultant difference inexcitation current between the heads. This difference in excitationcurrent, passed via output resistors 56 and smoothing capacitor 58,produces a direct current output signal which has polarity and magnitudeindicative of the magnitude and direction of the applied torque.

In accordance with one unique aspect of the present invention, ashereinbefore described, a shaft of suitable material is endowed in eachof two proximate bands with symmetrical, left and right handed helicalmagnetic easy axes. At least in the region of the bands, and morecommonly over its entire length the shaft is formed, at least at itssurface, of a material which is ferromagnetic and magnetostrictive. Thematerial must be ferromagnetic to assure the existence of magneticdomains and must be magnetostrictive in order that the orientation ofthe magnetization may be altered by the stresses associated with anapplied torque. Many materials are both ferromagnetic andmagnetostrictive. However, only those are desirable which also exhibitother desirable magnetic properties such as high permeability, lowcoercive force and low inherent magnetic anisotropy. In addition,desirable materials have high resistivity in order to minimize thepresence of induced eddy currents as a result of the application of highfrequency magnetic fields. Most importantly, favored materials mustretain these favorable magnetic properties following the cold workingand heat treating necessary to form them into suitable shafts havingappropriately high strength and hardness for their intended use.

It is true that many high strength steel alloys are ferromagnetic andmagnetostrictive. However, to varying degrees, the vast majority ofthese alloys experience a degradation in their magnetic properties as aresult of the heat treating necessary to achieve suitable hardness andstrength for the desired application. The most significant degradationis noted in those alloys hardened by carbon or carbides for which theconventional inverse relationship between mechanical hardness andmagnetic softness appears to have a sound basis. However, theperformance of even low carbon alloys such as AISI 1018 is found tosignificantly degrade with heat treating. The same is true formartensitic stainless steels, e.g., AISI 410, and highly alloyed steels,e.g., a 49Fe-49Co-2V alloy. It has been determined, in accordance withanother unique aspect of the present invention, that the nickel maragingsteels possess the unusual combination of superior mechanical propertiesand outstanding and thermally stable magnetic properties which give thema special suitability and make them particularly advantageous for use inall magnetoelastic torque transducers in which a magnetic field isapplied to ferromagnetic, magnetostrictive means and the change inpermeability caused by torque applied thereto is sensed to obtain anindication of the magnitude of the applied torque. This is the casewhether the ferromagnetic, magnetostrictive means is affixed to,associated with or forms a part of the surface of the torqued member andwhether or not the ferromagnetic, magnetostrictive means is endowed withbands of intentionally instilled magnetic anisotropy and irrespective ofthe number of bands which may be used.

The nickel maraging steels are, typically, extra-low-carbon, highnickel, iron-base alloys demonstrating an extraordinary combination ofstructural strength and fracture toughness in a material which isreadily weldable and easy to heat-treat. They belong to a loosely knitfamily of iron-base alloys that attain their extraordinary strengthcharacteristics upon annealing, by transforming to an iron-nickelmartensitic microstructure, and following cooling, upon aging in theannealed or martensitic condition. Thus, the alloys are termed"maraging" because of the two major reactions involved in theirstrengthening--martensitizing and aging. However, these steels areunique due to their high nickel and extremely low carbon content, whichpermits formation of an outstandingly tough martensite that can bestrengthened rapidly to extraordinarily high levels. Yield strengths upto and well beyond 300 ksi are available in these steels in the agedcondition.

Typical nickel maraging steels are alloys comprising 12-25% Ni, 7-13%Co, 2.75-5.2% Mo, 0.15-2.0% Ti, 0.05-0.3% Al, up to 0.03% C, balance Feand incidential amounts of other elements, such as Mn, Si, S, P, Cb. Themost popular and practically significant maraging steels, at least atpresent, are the 18% Ni steels which can be aged to develop yieldstrengths of about 200 ksi, 250 ksi and 300 ksi. These particularalloys, referred to as 18Ni200, 18Ni250 and 18Ni300 grade maragingsteels have typical compositions in the ranges 17-19% Ni, 7-9.5% Co,3.0-5.2% Mo, 0.1-0.8% Ti, 0.05-0.15% Al, up to 0.03% C, balance Fe andincidential amounts of other elements. Typically, the 18% nickelmaraging steels are heat treated by annealing at temperatures of 1500°F. and above for a sufficient time, e.g., one hour per inch ofthickness, to dissolve precipitates, relieve internal stresses andassure complete transformation to austenite. Following air cooling, the18% Ni steels are conventionally aged at 750°-1100° F., desirably900°-950° F., for 3 to 10 hours, depending upon thickness, usually 3-6hours. However, it has been found that satisfactory strengthcharacteristics and superior magnetic characteristics can be attained inalloys aged for as little as 10 minutes.

Other well known nickel maraging steels are cobalt-free 18% Ni maragingsteels as well as cobalt-containing 25% Ni, 20% Ni and 12% Ni maragingsteels. The 18% Ni-cobalt containing maraging steels are commerciallyavailable from a number of sources. Thus, such steels are obtainableunder the trademarks VascoMax C-200, VascoMax C-250, VascoMax C-300 andVascoMax C-350 from Teledyne Vasco of Latrobe, Pennsylvania; under thetrademarks Marvac 250 and Marvac 300 from Latrobe Steel Company ofLatrobe, Pennsylvania; under the trademark Unimar 300K fromUniversal-Cyclops Specialty Steel Division, Cyclops Corporation ofPittsburgh, Pennsylvania; and, under the trademark Almar 18-300 fromSuperior Tube of Norristown, Pennsylvania. The 18% Ni-cobalt freemaraging steels are commercially available under the trademarks VascoMaxT-200, VascoMax T-250 and VascoMax T-300 from Teledyne Vasco of Latrobe,Pennsylvania. Other high nickel steels which form an iron-nickelmartensite phase exhibit mechanical and magnetic properties which aresimilar to those of the more conventional maraging steels and which arealso substantially stable to temperature variations. Most notable amongthese is a nominally 9% Ni-4% Co alloy available from Teledyne Vascohaving a typical composition, in percent by weight, of 9.84 Ni, 3.62 Co,0.15 C, balance Fe. In addition, maraging steels of various other highnickel-cobalt compositions, e.g., 15% Ni-15% Co, are continously beingtested in efforts to optimize one or another or some combination ofproperties. Therefore, as used herein, the term "Ni maraging steel"refers to alloys of iron and nickel which contain from 9-25% nickel andwhich derive their strength characteristics from iron-nickel martensiteformation, as hereinbefore described.

In addition to their outstanding physical and strength characteristics,the nickel maraging steels have excellent magnetic properties which makethem outstanding for use as the magnetic material in non-contact torquetransducers. Thus, they have high and substantially isotropicmagnetostriction, in the range of 25 ppm±15 ppm, and do not exhibit aVillari reversal; high electrical resistivity; low inherent magneticanisotropies due to crystalline structure; high magnetic permeability;low coercive force, in the range 5-25 oersted; and, stability ofmagnetic properties with alloy chemistry. However, most important isthat their magnetic properties are only modestly, yet favorably,affected by strengthening treatments--indeed, their magnetic propertiesimprove with cold work and aging heat treatment. This characteristicdistinguishes the nickel maraging steels from all other high strengthalloys. Heretofore, it had been the conventional wisdom that the heattreatments needed to improve the mechanical and strength properties ofsteels were detrimental to their magnetic properties. For example,quench hardened steel alloys typically exhibit very low magneticpermeabilities and high coercive forces, a combination of unfortunatemagnetic properties which materially decrease the sensitivity of suchalloys to small magnetic fields and diminish or negate their usefulnessin torque transducers such as are contemplated herein. This isdemonstrably not the case with the nickel maraging steels. In accordancewith the present invention it has been determined that nickel maragingsteels get magnetically softer following cold work and the aging heattreatments to which they are conventionally subjected in order todevelop their extraordinary high strength characteristics. For example,the coercive force of an 18% Ni maraging steel in fact decreases whenaged at 900° F. for up to 10 hours. As a result the maraging steels canbe advantageously used in their aged condition, i.e., in a conditionwhere they exhibit maximum strength characteristics and substantiallythe same or improved magnetic characteristics.

Thus, the use of maraging steels as the magnetic material in amagnetoelastic torque sensor, particularly as the shaft material in adevice whose torque is to be sensed, obviates virtually all of theobjections heretofore made to using the device shaft as the magneticmember. The mechanical and strength properties of maraging steelssatisfy the mechanical properties requirements for most all shaftapplications while, at the same time, providing outstanding magneticproperties for its role in the torque sensor. Aging of the maragingsteels provides the high strength and high hardness needed for themechanical application without loss of magnetic permeability or increasein coercive force. Moreover, the conventional manner of heat treatingmaraging steel, including the initial solution anneal at temperatures inexcess of 1500° F., relieves internal stresses due to mechanical workingand most stresses due to inhomogeneities and crystal orientation, thusminimizing the amount of random magnetic anisotropy in a maraging steelshaft. When such heat treatment is combined with the creation, accordingto the present invention, of a pair of adjacent bands endowed withintentionally instilled magnetic stress anisotropy of a relatively largemagnitude, e.g., by stressing the shaft beyond its elastic limits withapplied stresses of a magnitude greater than the largest torque stressesanticipated during normal usage of the shaft, the contribution to totalmagnetic anisotropy of any random anisotropy in the shaft is indeednegligible.

It will be appreciated that the advantage of the nickel maraging steelsin magnetoelastic torque transducers can be realized by forming theshaft of the desired nickel maraging steel, by forming a region of theshaft of the desired nickel maraging steel and locating the annularbands within this region, or by surfacing with a nickel maraging steel ashaft formed of an alloy having mechanical properties suitable for theintended function of the shaft, i.e., applying over at least onecomplete circumferential region of suitable axial extent of the shaft asurfacing alloy of the desired nickel maraging steel and locating theannular bands within this region. Inasmuch as magnetic permeabilitysensing in accordance with the present invention is fundamentally asurface phenomena, the surfacing process need apply a circumferentiallayer of thickness not exceeding about 0.015 inches. The surfacingprocess selected may advantageously be selected from among the manyknown additive processes, e.g., electroplating, metal spraying,sputtering, vacuum deposition, ion implanatation, and the like.

In order to demonstrate the outstanding qualities of the maraging steelsas the magnetic material in torque transducers of the present inventionand to compare the performance of maraging steels with other highstrength steels, a torque transducer was assembled using a 12.7 mmdiameter cylindrical shaft having formed thereon a pair of axiallyspaced-apart bands endowed with helically symmetrical LH and RH magneticeasy axes. The bands each had an axial length of 12.7 mm and wereseparated by a 12.7 mm shaft segment. They were formed by knurling usinga 3/4-inch OD knurling tool having 48 teeth around the circumference,each tooth oriented at 30° to the shaft axis. The characteristics ofthis arrangement were sensed by positioning bobbins concentric with theshaft and axially aligned with the bands, each bobbin having amagnetizing and sensing coil mounted thereon. The magnetizing coils wereconnected in series and driven by an alternating current source having a10 KHz frequency output and a 200 mA peak driving current. The emfinduced in each of the sensing coils was separately rectified with therectified outputs oppositely connected to produce a difference signalwhich was displayed on a voltage display instrument. Four shafts wereemployed, identical in all respects except they were each formed ofdifferent materials. The composition of each shaft is set forth inpercent by weight hereinbelow:

T-250: 18.5 Ni; 3.0 Mo; 1.4 Ti; 0.10 Al; less than 0.03 C; no cobalt;balance Fe

SAE 9310: 0.08-0.13 C; 0.45-0.65 Mn; 3-3.5 Ni; 1-1.4 Cr; 0.08-0.15 Mo;balance Fe

416 SS: 11.5-13.5 Cr; 0.5 max Ni; 0.15 max C; 1.0 max Mn; 1.0 max Si;balance Fe

AISI 1018: 0.15-0.20 C; 0.6-0.9 Mn; 0.04 max P; 0.05 max S; balance Fe

In a first series of runs, the T-250 nickel maraging steel shaft wasused in the solution annealed, unaged condition as received fromTeledyne Vasco. Likewise, the other shafts were also used in theiraspurchased condition without further heat treatment. A known torqueloading was applied to each shaft under test and the output voltagesignal was recorded. The applied torque was increased from zero up to100 newton-meters (N-M). FIG. 5 is a graph of applied torque versusoutput d.c. voltage for each shaft. It is apparent that the sensitivityof the T-250 shaft in terms of magnitude of output signal for a giventorque loading was significantly greater than for the other shaftmaterials tested. In addition, the linearity of the output signal forthe T-250 shaft was extremely good over the entire torque range. Theother shaft materials appeared to be about equally insensitive, comparedto the T-250 shaft, to applied torque. None produced as linear a signalas the T-250 shaft, although each produced a reasonably linear signalover most of the torque range.

For the second series of runs, the T-250 nickel maraging steel shaft wasaged at about 900° F. for 30 minutes to improve the strength andhardness of the shaft. For consistency of testing, the other shafts wereheat treated in the same manner, after which each shaft was subjected toan applied torque from zero to 100 N-M and the output d.c. voltagerecorded. FIG. 6 is a graph of applied torque versus output d.c. voltagefor each shaft after heat treatment. It can be seen that once again thesensitivity of the T-250 shaft far exceeded the sensitivity of the othershafts and once again the T-250 output signal was linear over the entiretorque range. By comparison with FIG. 5 for the T-250 shaft in theunaged condition it is apparent that aging measurably improved thesensitivity of the shaft, indicating an enhancement of the magneticproperties of the maraging steel with aging. By contrast, thesensitivity of the SAE 9310 shaft did not appear to improve with thisheat treatment. Moreover, the linearity of the output signal clearlydegraded, particularly at higher applied torques. The sensitivity of theAISI 1018 shaft significantly improved at low applied torques but theimprovement began to abate at about 40 N-M and degraded thereafter. Thelinearity of the output signal for the aged AISI 1018 shaft was verypoor. For the 416 SS shaft, the sensitivity at low applied torquesimproved with heat treatment but significantly worsened at higherapplied torques. The linearity of the 416 SS output signal became worsewith heat treatment. It is noteworthy that notwithstanding the mixedresponse of the output signal to applied torque, heat treatmentadversely affected the mechanical and strength properties of the SAE9310, 416SS and AISI 1018 shafts. For example, following heat treatment,an applied torque of only about 50 N-M exceeded the elastic limit of theAISI 1018 shaft and the shaft permanently twisted.

FIGS. 5 and 6 graphically illustrate the signal response to appliedtorque using a relatively low, 10 kHz, a.c. excitation frequency. It hasbeen found that the output signal is directly proportional to andincreases approximately linearly with a.c. frequency. Tests show that at20 kHz, for example, a doubling of the output d.c. voltage signal isobtained. Depending upon the circuitry employed, a.c. frequencies in therange 1-100 kHz can advantageously be used to drive torque transducersof the present invention. Preferably, frequencies of 10-30 kHz, justabove the human audible range, are used in order to avoid whistling.Most desirably, the frequency is adjusted to about 20 kHz. Like itsresponse to frequency, the output d.c. signal also appears to bedirectly proportional to, more specifically to vary sigmoidally with,the drive current which, depending upon the frequency, can usefully bein the range 10-400 mA (peak). Generally, sufficient current is used toobtain a good signal at the chosen frequency and, desirably, to adjustthe signal hysteresis to zero over the entire applied torque range.

It is interesting to note that the sensitivity of a nickel maragingsteel shaft is markedly better than the sensitivities reported byworkers employing nonmagnetic shafts and adhesively affixing amorphousribbons thereto. From FIG. 6, it can be seen that according to thepresent invention an aged T-250 nickel maraging steel shaft transducer,having a shaft diameter of 12.7 mm, produces an output d.c. signal of0.9 volts at an applied torque of 60 N-M using an a.c. frequency of 10kHz and an exciting current of 200 mA and employing exciting coilshaving 100 turns each and sensing coils having 500 turns each, asensitivity of 0.015 V/N-M. By comparison, Sasada et al, in the paper"Noncontact Torque Sensor", presented at the 11th Annual IEEE IndustrialElectronics Society Conference (Nov. 18-22, 1985) reports, for anamorphous ribbon torque sensor, an output d.c. signal of 35 mV at anapplied torque of 10 N-M using an a.c. frequency of 20 kHz, an excitingcurrent of 120 mA, exciting coils having 220 turns each and sensingcoils having 80 turns each and a shaft diameter of 12 mm. Inasmuch assensitivity is directly proportional to a.c. frequency, exciting currentand number of turns on the exciting and sensing coils and inverselyproportional to the cube of the shaft diameter, the Sasada et alsensitivity corrected to an equivalent basis as that shown in FIG. 6hereof is 0.007 V/N-M. In other words, the torque transducer of thepresent invention is more than twice as sensitive as the amorphousribbon torque sensor of Sasada et al.

INDUSTRIAL APPLICABILITY

The unique and improved magnetoelastic torque transducers of the presentinvention are broadly useful for the sensing and measurement of torquein members of all types and sizes, whatever may be the device or fieldof application in which the member operates. It is universally acceptedthat torque is an absolutely fundamental parameter in the control ofsystems having rotating members. Sensing the instantaneous torqueexperienced by a rotating member and generating an electrical current inresponse thereto which bears a known relationship to the torque allowsthe early diagnosis of incipient problems or the control, viamicroprocessor or otherwise, of the engine, machine, motor, etc. whichdrives the rotary member.

Applications for the torque transducers of the present invention can befound in virtually every device having a rotating member. There alreadyis a demand for sensitive, responsive, and inexpensive magnetic torquesensors for monitoring torque in engines and power drives to improveoverall performance and fuel economy, control exhaust emissions andmodulate transmission ratios; in marine propulsion systems to detect andcorrect reduced output from the propulsion machinery and the effects ofhull fouling and propeller damage; in helicopter turbines to avoidoverloading and to detect power loss caused, for example, by sand orsalt spray. There is also a demand for torque transducers such as areprovided in accordance with the present invention for controlling heavyindustrial machinery of all types, e.g., pulp grinders for maintainingfiber quality, paper-making machines, and the like, as well as for usein consumer home and commercial appliances, e.g., food mixers andprocessors. In addition, the need for small, inexpensive, sensitive,reliable torque sensors has been noted in such diverse applications asmachine and hand tools, robotics, information devices, industrialmeasuring instruments, weighing systems of various kinds, electronicpower assisted power steering, and vehicular traction balancing.

One application for the magnetoelastic torque transducers of the presentinvention which is particularly promising in view of the potentialcontribution of these transducers to energy conservation, environmentalcleanliness and safety and because it directly affects so many peopleand businesses is its use on internal combustion engines and associatedengine power drives. The torque sensor of the present invention iscapable of recovering the torque signature of an engine over a wideenough bandwidth to discern salient details of important torquecontributing events at all points between idle and the top operatingspeed of the engine. Torque sensing in an accurate and cost effectivemanner enables early diagnosis of incipient problems due to thefunctional condition of the engine, helps to avoid unanticipatedfailures that might limit the servicability of the vehicle at criticaltimes and improves and/or controls the performance and economy of theengine and its power drive.

Primary power for the propulsion and other essential functions of modernvehicles is obtained from the rotating output shaft of an internalcombustion engine. Regardless of the type of engine the power actuallydelivered by this shaft to the vehicle is the numerical product of onlytwo parameters: rotational speed and transmitted torque. Of the two,torque is the intensive parameter since rotational speed is itselfconsequential to the internally developed torque of the engine. It isthe magnitude of available torque that sets the limits on vehicleacceleration, its speed on grade and other mobility and performancefactors. The successful use and enjoyment of the vehicle depends,ultimately, on the ability of its engine to deliver the functionallyrequired torque throughout its operational range of speeds.

Except for the situation where a turbine engine is driving a constantload, the torque transmitted through an engine output shaft fluctuatesrapidly. These fluctuations reflect both the cyclic variations in thetorque developed by the engine and transient variations in the torqueimposed by vehicle loads. In piston engines, torque is developed by eachcylinder only during its power stroke. Multicylinder engines attain somecontinuity of developed torque by the overlap of phased power strokesfrom each cylinder. While cyclic variations in output torque are alsoreduced thereby, and further reduced by the combined inertia of theengine's internal moving parts, the strongly impulsive nature of eachcylinder's developed torque is still transmitted through the outputshaft. Cyclically stimulated torsional vibrations together with thechanging accelerations of linked reciprocating parts contributeadditional time varying torque components. The magnitude and even thedirectional sense of this torque is further influenced by variations inoperational conditions of the vehicle, e.g., throttle settings, gearpositions, load pick-up, road surface inclination and roughnessfeatures.

Although the torque on the engine output shaft represents thesuperposition of contributions from this multiplicity of sources, manyare strongly interdependent and their combination forms an effectivesignature characterizing the engine's performance. Salient features ofthis signature would clearly correlate with specific engine events,e.g., cylinder firings. The absence of a normal feature, its alterationor the development of new features would be a reflection of adysfunction. The nature and extent of the abnormality would besymptomatic of specific engine or drive line difficulties. While manyengine problems are also detectable by their symptomatic effects onoverall performance and/or more objectively measurable quantities (e.g.,manifold pressure, compression, noise signature, exhaust gas analysis),none are as sensitively quantified as torque to the individual eventswhich together characterize proper engine function. Since torque is theeffective product of the engine, no measurements of indirectly relatedparameters can so clearly identify the source of inadequate productionas can the measurement of torque itself. Conventional methods ofrecovering torque data, whether by dynamometer or from measurements ofunloaded engine acceleration and deceleration by procedures involvingstepped changes in fuel flow and/or ignition interruption, determineonly average values and lack the detail needed for clear diagnosis andcontrol. Recovery and analysis of the information contained in thetorque signature of the engine output shaft enables diagnosis ofincipient problems, helps to avoid unanticipated failures that mightlimit the servicability of the vehicle at critical times and improvesand/or controls the performance and economy of the engine and its powerdrive. The key to the problem is the recovery of enough torqueinformation for a meaningful analysis.

In a 12 cylinder, 4 stroke engine operating at 4000 rpm there are 400power strokes and (at least) 1600 valving events (openings or closings)every second. Turbine engines run with far smoother power input but atspeeds up to 500 revolutions per second. To be capable of discriminatingimportant details of these salient events, the torque sensing systemmust have a reasonably flat frequency response up to at least severaltimes the maximum event rate, i.e., in the vicinity of 5 kHz. Thefrequency response must also extend downward to zero Hz to faithfullycapture the steady state torque components imposed by the vehicle loads.

Although that full bandwidth is obviously desirable for maximum utilityas a diagnostic tool, the information contained in the low frequencyspectrum, up to 10 Hz, accurately describes the engine's overallresponse to control (input) and load (output) changes. Not only canvariations in performance be objectively evaluated from this informationbut it also has potentially prime utility in another area, control ofthe engine and associated power drive.

A torque sensor having 5 kHz bandwidth capability cannot be positionedarbitrarily. While torque is applied to the engine shaft by contactforces at discrete locations, it is transmitted axially by continuousstress distributions. Transient torque events are not transmittedinstantaneously nor do they remain unaltered along the shaft. The finiteelasticity and inertia of real shaft materials combine to limit thetransmittable rate of change of torque. Steep transients triggeroscillatory exchanges of elastic and kinetic energy (stress waves) whichtravel with material and mode dependent characteristic velocities alongthe shaft. The fidelity of the transmitted torque is further reducedwith distance from its source by the accumulated dissipative effects ofinternal and external friction. The sensor must therefore be locatedclose enough to the source(s) to avoid losing the desired torqueinformation either by attenuation or in background "noise" composed ofcomplex combinations of interfering and reflecting stress waves.

Important sensor requirements are that it be small, at least in thedimension parallel to the shaft axis, that it be rugged and that it befree from deteriorating effects of use or time such as wear, corrosionor fatigue. The sensor should be amenable to performance verificationand calibration, especially in the event of repair or replacement ofparts of the torque sensing system, including the engine shaft. Itshould have neglible impact on engine and drive line manufacturability,operation and maintenance and, under no circumstances should the failureof the torque sensor have any contingent consequences which interferewith the otherwise normal operation of the vehicle.

Another promising and very different application for the magnetoelastictorque transducers of the present invention is their use in hand tools,particularly tools such as screwdrivers, wrenches, and the like, e.g.,for applying predetermined and pre-set torque to fasteners. Torque toolshave become widely used and very important as industrial production andquality control tools for effecting proper tension on bolts, screws, andthe like, to insure optimum tightness in the assembly of products ofwhich these fasteners are a part. It is well known that fasteners havemechanical stress limits which, if exceeded, cause them to weaken orbreak. Therefore, torque tools for applying or checking the torquelevels in fasteners have become indispensable in maintaining qualitycontrol by eliminating the guesswork which would otherwise be associatedwith conventional wrench or screwdriver tightening of assembledproducts.

Most typically, hand operated torque tools used for assembly ordisassembly are of the type which can be pre-set to the desired torquelevel. When the desired torque is reached, the tool either releases,thereby preventing further torque from being applied, or signals theuser, as by a visual indication or audible signal, that the desiredtorque has been reached. Torque tools which automatically release whenthe desired torque has been reached have the advantage that overtorquingis not possible, despite inattention of the user. However, therepetitive releasing of the tool on each and every fastener each timethe desired torque is reached is disconcerting and annoying,particularly to the skilled worker, wastes time and, therefore,contributes to inefficiency. Tools which provide a visual indicationthat the desired torque has been reached, such as by providing anindicator dial or lighting a bulb, avoid many of the shortcomings ofautomatic release tools. However, in many situations the location of thefastener is such that the user is physically unable to simultaneouslyapply the torque and view the visual indicator. Therefore, the use of anaudible signal to indicate when the desired torque has been reached ismuch to be preferred.

In accordance with the present invention there is provided a hand toolfor applying torque to a fastener which, employing the magnetoelastictorque transducers of the present invention, constantly senses thetorque being applied and provides an audible signal when a pre-settorque level has been attained. The torque tool of the present inventionis well balanced, comfortable to use, rugged and able to sustain abuse,convenient to adjust and maintain, accurate and reliable, andinexpensive to fabricate.

Referring to FIGS. 10, 11 and 12, one embodiment of the torque tool ofthe present invention is shown generally at 200. Tool 200 includes agripping handle 202 non-rotatably coupled to shaft 204 such that shaft204 rotates with handle 202 as a turning moment is applied thereto.Likewise, the free end of shaft 204 is adapted to non-rotatably receivea fastener engaging means, such as a screwdriver bit 206, such that theturning movement applied to handle 202 is transmitted via shaft 204 tobit 206. Although the torque tool is illustrated herein as a torquescrewdriver, it should be understood that the present invention isequally applicable to any similarly operated torque tool. Therefore, thefastener engaging means is not intended to be limited to theconventional flat blade screwdriver bit illustrated, but also embracesother suitable bits such as, for example, Phillips head, Allen head,socket head, and the like.

Handle 202 comprises a hollow, generally cylindrical housing which isadvantageously made of electrically non-conducting material such asplastic, wood, and the like, having flats formed on the outer surfacethereof to facilitate gripping. Closing the topmost or open end 212 ofhandle 202 is a removable cap 208 having an integral externally threadedsleeve 210 for screw threadably engaging and being held securely bycorresponding threads formed within the open end 212 of handle 202.Handle 202 advantageously tapers at its innermost end 214 and includesbore 216 formed in end 214 for communicating with the hollow interior ofthe handle. Bore 216 is preferably non-round, e.g., square, rectangular,hexagonal, etc. in cross-section, desirably containing a plurality offlat surfaces, for engaging corresponding flat surfaces of shaft adapter218 which may be press fit within bore 216. One end portion 222 of shaft204 is removably received within bore 220 of adapter 218 and interlockedtherein against rotation relative to the adapter. This may beaccomplished in many well known ways, such as by providing flats onshaft end portion 222 which engage corresponding flats formed in bore220 while the diameters of end portion 222 and bore 220 are sufficientlyclose that the shaft fits tightly within the bore. The opposite or freeend of shaft 204 has a bit receiving socket 224 rigidly affixed theretoin which screwdriver bit 206 is received. Socket 224 is of aconventional type and is adapted to securely hold the shank portion 226of bit 206 therewithin in such tight engagement that the bit will notrotate relative to the socket. One means for accomplishing this is toutilize a conventional hexagonal bore, ball and spring socket in which ametal ball 228 partially protrudes through the socket wall into thesocket bore and is retained in place by a spring clip 230 extendingcircumferentially around the socket wall for biasing the ball into thesocket. The clip forces the ball inwardly into tight engagement with thesidewalls of hexagonal bit shank portion 226 to securely hold the bitwithin the socket while the flats of the hexagonal socket bore engagethe flats of the hexagonal shank portion 226 to prevent relativerotation.

A magnetoelastic torque transducer in accordance with the presentinvention is formed on shaft 204 in a region intermediate its ends and,desirably, closely adjacent end portion 222. Shaft 204 is formed offerromagnetic and magnetostrictive material or at least has aferromagnetic and magnetostrictive region in which a pair of axiallyspaced apart circumferential bands 232, 234 are formed. The bands areendowed, as hereinbefore described, with respectively symmetrical,helically directed magnetic stress anisotropy. In an exemplary form ofthe invention, the shaft is formed of Unimar 300, an 18% nickel maragingsteel, which has been machined from its 0.250 inch original diameter toprovide two axially spaced apart regions 0.30 inch long and 0.200 inchin diameter, separated by an intermediate shaft portion 236 having anaxial length of 0.15 inch and provided with opposite flats for clamping.Following machining the shaft is annealed at 815° C. to remove internalstresses formed during machining and is then air cooled. The shaft isclamped in a vise along intermediate portion 236 and end 222 of theshaft is twisted in a clockwise direction to provide a torsionaloverstrain in one of the reduced diameter regions, corresponding to band232. The shaft is then repositioned in the vise so that the opposite,free end is located where end 222 was originally and the free end istwisted in a counterclockwise direction to provide an oppositelydirected torsional overstrain in the other reduced diameter region,corresponding to band 234. Following twisting, the shaft is aged 10minutes at 480° C.

A magnetic discriminator for sensing the permeability change of bands232,234 upon application of a torque to shaft 204 via handle 202 andexemplary indicator circuitry are illustrated in FIG. 12 (excludingconventional voltage regulator circuitry to assure a constant V_(cc)).The discriminator includes a pair of coils 238,240 formed on anon-conducting, e.g., cardboard sleeve 242, and positioned in axiallocations corresponding to bands 232 234. A coil cover 272 (see FIG. 11)is desirably provided for protecting the coils when the tool is in use.The remainder of the discriminator circuitry as well as the audibleindicator and the indicator circuitry are housed within the hollow boreof handle 202. A conducting metal sleeve 244, closed at one end forhousing battery 246 and providing spring contact 248 as a first batteryterminal contact, is positioned adjacent end 212 of handle 202. Axiallyspaced from battery 246 within handle 202 by non-conducting spring 250is the encapsulated electronics package 252 of the transducer comprisingthe multivibrator and indicator circuitry illustrated in FIG. 12. Asecond battery contact 254 and a circumferential sleeve contact 255 areformed on the rearmost portion of the encapsulated package adjacent andfacing battery 246. A battery connect/disconnect switch 256 is locatedin a recess of handle cap 208 and is mechanically cammed to detentmechanism 258 to slide battery 246 and sleeve 244 forward, in the switchON position, into electrical contact with contacts 254 and 255. In theswitch OFF position, detent mechanism 256 allows spring 250 to slidebattery 246 and sleeve 244 rearwardly out of electrical contact withcontacts 254 and 255. Cables 260, 262, 264 extend forwardly fromelectronics package 252 via wire harness keyway 268 in shaft adapter 218to electrically connect coils 238,240 to the corresponding circuitrywithin electronics package 252. An applied torque calibration knob 270for setting the torque at which audible indicator 266 will signal, ispositioned at a convenient location along the outer surface of handle202. Knob 270 is coupled to a potentiometer P₁ in the multivibrator andindicator circuitry.

In operation of the torque tool of the present invention an appropriatebit for engaging the fastener to be torqued is inserted in socket 224,switch 256 is operated to the ON position and knob 270 is turned to theappropriate torque setting for the particular fastener. Adjustment ofknob 270 alters the resistance of potentiometer P₁ which alters thecircuit characteristics and, correspondingly, the applied torque atwhich indicator 266 produces an audible signal. Bit 206 is placed inoperative engagement with the fastener to be tightened (or loosened) andhandle 202 is rotated in the appropriate direction, thus rotating thefastener. As the fastener tightens, the resistance to turning increasesrequiring application of additional torque to overcome the resistance.In the manner previously described herein, the torque applied to handle202 and transmitted to bit 206 is also present in shaft 204. When torqueis applied to shaft 204 the magnetic permeability of one band 232,234increases while the permeability of the other decreases. In themultivibrator circuit shown, only one of the transistors Q₁, Q₂ conductsat a time, thus allowing a square wave voltage to create a cyclicallytime varying magnetic field for application to the bands 232,234 withthe result that the inductance of one coil 238,240 increases while theinductance of the other decreases. This difference in inductanceproduces different voltage signals V₁, V₂ which enter the operationalamplifier OA 1, serving as a comparator, for comparing V₂ and V₁. If V₂exceeds V₁, then V_(out) will be positive and indicator 266 will producean audible signal. If V₁ exceeds V₂, then V_(out) will be zero and noaudible signal will be produced.

Variable potentiometer P₁ effectively acts to increase V₁ while theapplied torque increases V₂. Therefore, if it is desired to apply alarge torque to a fastener, knob 270 is operated to increase theresistance in the R₁ circuit, which has the effect of increasing V₁.With V₁ large, V₂, the voltage indication of applied torque, must evenbe larger in order for V₂ to exceed V₁ to produce an audible signal.Since it requires a large applied torque for V₂ to be large, it followsthat with knob 270 adjusted to increase the resistance contribution ofP₁ to the R₁ circuit, the audible signal will not be produced until theapplied torque is large. Conversely, with knob 270 adjusted to minimizethe resistance contribution of P₁ to the R₁ circuit, a smaller appliedtorque will produce an audible signal.

The torque tool of the present invention has been described herein as aself-contained, stand alone tool which includes, in a single hand heldunit, the torque transducer, all necessary electronics, power source andtorque indicator. However, in many industrial applications it may bemore desirable to distribute the operational and functional featuresbetween a hand held portion and a separate stationary portion. Thus,functionally, the very same torque tool may be embodied in a wirelesssystem combining a hand tool containing the torque transducer, a powersource and such electronics as is required to radiate a signal,modulated by the torque, e.g., via infra red or radio frequency, to anearby receiver in which is housed such additional electronics as arerequired to provide the torque indication. In still another form, thesame functional torque tool may be embodied in a wired system combininga hand tool containing the torque transducer with minimal localelectronics which is connected via wire to a control unit containing thepower source and such other electronics as are required to provide thetorque indication.

The context is clear, whether for engines, power drives, machine or handtools, or other uses, a suitable torque sensor should be an unobtrusivedevice that is difficult to abuse and is capable of reliably recoveringmuch of the torque information available on the torqued shaft. None ofthe heretofore contemplated state of the art torque transducers can meetthese requirements. However, the magnetoelastic torque sensors of thepresent invention appear eminently suitable in all respects and will,for the first time, make inexpensive, reliable and sensitive torquesensors available for commerical implementation.

I claim:
 1. A hand tool for applying torque to a fastener includingfastener-engaging means, handle means to which torque is hand-appliedand torque carrying means operatively coupled with said handle means andsaid fastener-engaging means for transmitting the hand-applied torque tothe fastener, said torque carrying means including a magnetoelastictorque transducer, said torque carrying means comprising a member havinga ferromagnetic and magnetostrictive region formed of nickel maragingsteel, said transducer comprising:a pair of axially spaced-apart annularbands defined within said region, said bands having respectivelysymmetrical right and left hand helically directed residual stresscreated magnetic anisotropy, each said band having at least onecircumferential region which is free of residually unstressed areas overat least 50% of its circumferential length; means for applying acyclically time varying magnetic field to said bands; means for sensingthe change in permeability of said bands caused by said applied torque;means for converting said sensed change in permeability to an electricalsignal indicative of the magnitude of the torque applied to said member;and indicator means responsive to said electrical signal for providingan indication that a predetermined torque has been applied to saidfastener.
 2. A tool, as claimed in claim 1, wherein each said band hasat least one circumferential region which is free of residuallyunstressed areas over at least 80% of its circumferential length.
 3. Atool, as claimed in claim 1, wherein each said band has at least onecontinuous circumferential region which is free of residually unstressedareas.
 4. A tool, as claimed in claims 1, 2 or 3, wherein said regioncomprises the surface of said member.
 5. A tool, as claimed in claims 1,2 or 3 wherein said region is rigidly affixed to the surface of saidmember.
 6. A tool, as claimed in claims 1, 2 or 3 wherein said region isformed of 18% Ni maraging steel.
 7. A tool, as claimed in claims 1, 2 or3 wherein the magnetic easy axes in said bands are oriented,respectively, at angles of ±20°-60° to the axis of said member.
 8. Ahand tool for applying torque to a fastener including fastener-engagingmeans, handle means to which torque is hand-applied and torque carryingmeans operatively coupled with said handle means and saidfastener-engaging means for transmitting the hand-applied torque to thefastener, said torque carrying means comprising a member including amagnetoelastic torque transducer, ferromagnetic, magnetostrictive meansassociated with said member for altering in magnetic permeability inresponse to the application of torque to said member, means for applyinga cyclically time varying magnetic field to said ferromagneticmagnetostrictive means, means for sensing the change in permeabilitycaused by said applied torque, and means for converting said sensedchange in permeability to an electrical signal indicative of themagnitude of the torque applied to said member, and indicator meansresponsive to said electrical signal for providing an indication that apredetermined torque has been applied to said fastener, the improvementcomprising forming said ferromagnetic, magnetostrictive means fromnickel maraging steel.
 9. A tool, as claimed in claim 8, wherein saidferromagnetic, magnetostrictive means comprises of the surface of saidmember.
 10. A tool, as claimed in claim 8, wherein said ferromagnetic,magnetostrictive means is rigidly affixed to the surface of said member.11. A tool, as claimed in claims 8, 9 or 10 wherein said ferromagnetic,magnetostrictive means is formed of 18% Ni maraging steel.
 12. A tool,as claimed in claims 8, 9 or 10 wherein said ferromagnetic,magnetostrictive means includes a pair of axially spaced-apart annularbands defined therewithin, said bands having respectively symmetricalright and left hand helically directed residual stress created magneticanisotropy, each said band having at least one circumferential regionwhich is free of residually unstressed areas over at least 50% of itscircumferential length, said applying means applying said magnetic fieldto said bands, said sensing means sensing the change in permeability ofsaid bands caused by said applied torque.
 13. A tool, as claimed inclaim 12, wherein each said band has at least one circumferential regionwhich is free of residually unstressed areas over at least 80% of itscircumferential length.
 14. A tool, as claimed in claim 12, wherein eachsaid band has at least one continuous circumferential region which isfree of residually unstressed areas.
 15. A tool, as claimed in claim 12,wherein the magnetic easy axes in said bands are oriented, respectively,at angles of ±20°-60° to the axis of said member.
 16. A tool, as claimedin claim 8, wherein at least a portion of said ferromagnetic,magnetostrictive means is endowed with helically directed residualstress created magnetic anisotropy, at least one circumferential regionof said portion being free of residually unstressed areas over at least50% of its circumferential length, said applying means applying saidmagnetic field to said endowed portion and to an area of said member notso endowed, said sensing means sensing the permeability differencebetween said portion and said area resulting from the application oftorque to said member, said converting means converting said sensedpermeability difference to an electrical signal indicative of themagnitude of the applied torque.
 17. A tool, as claimed in claim 16,wherein said circumferential region is free of residually unstressedareas over at least 80% of its circumferential length.
 18. A tool, asclaimed in claim 16, wherein said portion has at least one continuouscircumferential region which is free of residually unstressed areas. 19.A hand tool for applying torque to a fastener includingfastener-engaging means, handle means to which torque is hand-appliedand torque carrying means operatively coupled with said handle means andsaid fastener-engaging means for transmitting the hand-applied torque tothe fastener, said torque carrying means including a magnetoelastictorque transducer, said torque carrying means comprising a member havinga ferromagnetic and magnetostrictive region, said transducercomprising:a pair of axially spaced-apart annular bands defined withinsaid region, said bands having respectively symmetrical right and lefthand helically directed residual stress created, controlled magneticanisotropy of sufficiently large magnitude compared with the randommagnetic anisotropy in said member that the contribution to totalmagnetic anisotropy of any random anisotropy is negligible; means forapplying a cyclically time varying magnetic field to said bands; meansfor sensing the change in permeability of said bands caused by saidapplied torque; means for converting said sensed change in permeabilityto an electrical signal indicative of the magnitude of the torqueapplied to said member; and indicator means responsive to saidelectrical signal for producing an indication that a predeterminedtorque has been applied to said fastener.
 20. A tool, as claimed inclaim 19, wherein said region is formed of nickel maraging steel.
 21. Atool, as claimed in claims 19 or 20, wherein each said band has at leastone circumferential region which is free of residually unstressed areasover at least 50% of its circumferential length.
 22. A tool, as claimedin claim 21, wherein said circumferential region is free of residuallyunstressed areas over at least 80% of its circumferential length.
 23. Atool, as claimed in claim 21, wherein each said band has at least onecontinuous circumferential region which is free of residually unstressedareas.
 24. A tool, as claimed in claim 21, wherein said region comprisesthe surface of said member.
 25. A tool, as claimed in claim 21, whereinsaid region is rigidly affixed to the surface of said member.
 26. Atool, as claimed in claims 19 or 20, wherein said region comprises thesurface of said member.
 27. A tool, as claimed in claims 19 or 20,wherein said region is rigidly affixed to the surface of said member.28. A tool, as claimed in claim 1, 8 or 19, including means forpresetting the predetermined torque level at which said indicator meansprovides said indication.
 29. A tool, as claimed in claim 1, 8 or 19,wherein said indicator means comprises audible indicator means.
 30. Atool, as claimed in claim 1, 8 or 19, wherein said means for applying acyclically time varying magnetic field includes multivibrator circuitryfor producing a substantially square wave voltage signal.